The present invention relates to an electrophotographic developer, and more particularly to an improvement in a ferrite carrier as a toner carrier in the two-component developer.
A cascade development method and a magnetic brush development method are known as methods for electrophotographic development where the so called one-component developer and two-component developer are used as developers. The toner carrier of the so called two-component developer requires an appropriate triboelectric property to attract toner particles, particles that are high enough in density and strength to withstand breakup and are high in flowability, uniform particle size, a constant surface state stable in humidity various other conditions high tensile strength high, compression strength, etc., and appropriate magnetic properties such as saturation magnetization, permeability, coercive force, etc.
Various materials have been used for the toner carrier, and now iron powder is most widely used. Iron powder carrier is used generally after an appropriate surface treatment, but the surfaces of iron powder particles undergo physical or chemical change when it is used for a long time, and consequently toners remain on the carrier surfaces or the carrier becomes so sensitive to the humidity of the surrounding atmosphere as to lose good image quality. Thus, the life of carrier is shortened. These are disadvantages of iron powder carrier.
Ferrite has been proposed as a toner carrier having such disadvantages of iron powder carrier (e.g. U.S. Pat. No. 3,929,657). However, known electrophotographic ferrite carriers are mainly the so called spinel type ferrite, which have not always been found satisfactory with respect to image characteristics or life according to the results of copy-testing the ferrite of such type prepared by the present inventors as a ferrite carrier, and a better toner carrier has been still in demand.
The present invention has been established to meet such demand.